Add non-intrusive auto-updater

Tauri updater + process plugins. Checks Gitea Releases endpoint a few
seconds after launch; on a newer signed version shows a dismissible
bottom-right banner with download progress and auto-relaunch. Public
signing key in tauri.conf.json; release flow documented in RELEASING.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -24,3 +24,8 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 tauri-plugin-dialog = "2.7.1"
 
+# Auto-updater (desktop only — checks Gitea Releases for a signed new version)
+[target.'cfg(not(any(target_os = "android", target_os = "ios")))'.dependencies]
+tauri-plugin-updater = "2"
+tauri-plugin-process = "2"
+